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Skeletal Muscle: Structure & Function

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Skeletal Muscle:Structure & Function

• The human body contain over 400 skeletal muscles, whichconstitute 40-50% of the total body weight.

• 3 functions:

– Force generation for locomotion & breathing– Force generation for postural support

– Heat production during periods of cold stress

• Skeletal muscles are attached to bones through a connectivetissue called tendon

• One end of the muscle is attached to the bone that does notmove (origin), while the opposite end in fixed to a bone(insertion) that is move during muscular contraction.

• Muscles that decrease joint angle are call flexors, while musclethat increase joint angle are called extensors

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Structure of Skeletal Muscle

• Skeletal muscle is composed of several kind of tissues,include muscle cells, nerve, blood and various type of connective tissue

• Individual muscle are separate from each other and in

position by a connective tissue call Fascia• There are 3 separate layer:

– Epimysium (outer layer)

– Perimysium (middle layer)

– Endomysium (inner layer)

• The cell membrane surrounding the muscle cell is calledthe Sarcolemma.

• Beneath the sarcolemma lies the Sarcoplasm (cytoplasm)which contain myofibrils.

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Structure of Skeletal Muscle

• In general, myofibrils composed of two type of proteinfilaments:– Myosin (thick filament)

– Actin (thin filament)

• Both play an important role in the regulation of contractileprocess.

• Myofibrils can further subdivided into individual segmentcalled sarcomeres.

• Sarcomere are divided from each other by a thin sheet of 

structural protein called a Z line• Myosin filaments are located primarily in the dark portion

of sarcomere (A band ), while actin filament occur in thelight portion (I band). In the center of the sarcomere thereis a portion of the myosin filament with no overlap of actin,

called H zone.

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Structure of Skeletal Muscle

• Within a sarcoplasm, there is a membranous channel called

sarcoplasmic reticulum (SR)

• SR – storage site for calcium• Another set of membranous channel is transverse tubules (t

tubules)

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Muscular Contraction

• The process of muscular contraction is best explained

by the sliding filament model of contraction

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Sliding Filament ModelRelaxed Muscle Actin & myosin filaments in relaxed muscle

and contracted muscle are the same length

Contracting

muscle

During contraction, actin at each end of the

sarcomeres slide past the myosin toward

each other. Z disk are closer & sarcomeresshorten.

Contracting

muscle

As the actin slide over the myosin, the H

zone and the I band narrow. The A nab do

not narrowFully contracted

muscle

In a fully contracted muscle the end of the

actin overlap and the H zone disappears

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Fiber Types

• Muscle fibers can be classified into 2 categories:

– Type I - Slow twitch (ST) (slow oxidative)

– Type II - Fast twitch (FT)

• Type IIa (fast oxidative glycolytic)

• Type IIb or IIx (fast glycolytic)

Max Short

velocity

Type I Type IIa Type IIb/IIx

0

4

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Characteristics of Fiber Types

Fast Fibers Slow Fibers

Characteristic Type IIb / IIx Type IIa Type 1

Number of 

mitochondria

Low High/Mod High

Resistance to

fatigue

Low High/Mod High

Predominant energy

system

Anaerobic Combination Aerobic

ATPase activity Highest High Low

Vmax (speed of 

shortening)

Highest Intermediate Low

Efficiency Low Moderate High

Specific tension High High Moderate

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Fiber Type & Performance

Sports % Slow Fibers

(Type 1)

% Fast Fibers

(Type Ila & Ilb)Distance runner 70-80 20-30

Track Sprinters 25-30 70-75

Non athletes 47-53 47-53

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Alteration of Muscle Fiber Type

by Exercise Training

• In the past, researchers have concluded that endurancetraining does not result in the conversion of FT to STfiber.

• By contrast, recent investigations using improvedtechnique have shown that rigorous exercise trainingresults in alteration of muscle fiber types.

• Long duration exercise training (90 min/day; 10 week)

is capable of promoting a Type II to Type I fiber• However, the resistance training –induced changes in

fiber type are often small & do not result in completeconversion to Type IIb to Type I fiber

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Age Related Changes in

Skeletal Muscle Aging associated with a loss of muscle mass (atrophy)

• 25 to 50 years – 10% loss

• 50 to 80 years – additional 40% loss

• Aging results in loss of FT fibers (esp. Type IIb)

• Loss of muscle in lower limb is more pronounced in

older adults

• However, regular exercise (PRT) can improve muscular

endurance and strength but cannot completely

eliminate the age related loss in muscle mass

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Muscle Actions

3 type of actions:• Concentric

– Shortening of muscle

– Dynamic action

• Static

– Muscle generating force but it length remains

static (unchanged)

– Isometric action• Eccentric

– Lengthening of muscle

– Dynamic action

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Generation of Force

Depends on:

– The number of motor units activated• More motor unit – more force

– The type of motor units activated• FT generate more force

– The size of the muscle• Larger muscle generate more force

– The muscle initial length when activated

• Increasing or decreasing the muscle length beyond 20% reduce forceproduction

• However, in general, the amount of power generated by a musclegroup increases as a function of movement velocity.

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Generation of Force

Depends on:

– The angle of the joint• E.g., biceps brachii – best joint angle is 1000 to lift 100lb force

– The muscle’s speed of action (force-velocity/ power-velocity)

• During concentric actions, maximal force decreases at higher speed.

• Fast eccentric actions allow max application of force

• Higher % FT – velocity or speed is greater• However, in general, the amount of power generated by a muscle

group increases as a function of movement velocity.

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Receptors in Muscle

• Chemoreceptor– Send info to CNS in response to changes in

muscle pH, [potassium], changes in O2 & CO2 tensions.

• Golgi Tendon Organs– Provide CNS with feedback concerning the

tension developed by the muscle

– Serve as safety devices that help preventexcessive force during muscle contraction

• Muscle Spindles– Provide sensory information concerning relative

muscle length

– “Length Detector”